When a patient has an obstruction of the ureter, it is common to relieve the obstruction with a ureteral stent to enable urine to pass from the kidney to the bladder. Typically, the stent extends from the kidney to the bladder. In some cases, the stent has a retention configuration, such as a pigtail, at its ends in the kidney and the bladder.
A common case of ureteral obstruction is the ureteral stone, while cancerous tumor or a feature of the anatomy that allows ureter kinking can also produce ureteral obstruction.
Another occasion for use of a ureteral stent is after lithotripsy has been performed to break up a stone. A stent may be placed to allow fragments of stone to pass from the body and enable the ureter to heal.
Ureteral stents may be introduced to the body either percutaneously in an antigrade fashion, using, for example, an adaptation of the Seldinger technique, or cystoscopically in a retrograde fashion. The stents positioned in the bladder through a cystoscope are passed into the ureter using direct vision through the endoscope positioned in the bladder. For thus placing the stent there are two common methods. One is the so-called over-the-wire placement method. A guidewire of sufficient stiffness and maneuverability is inserted into the ureter under endoscopic guidance. When access past the ureteral obstruction to the kidney is achieved, the stent is introduced to the ureter over the wire by a pusher catheter acting on the trailing end of the stent. The common guidewire size that urologists prefer is 0.038 inch diameter, selected to be stiff enough to negotiate past the obstruction, but small enough to enable passage of a small stent over it.
The second common endoscopic placement method for ureteral stents, which omits the prior step of placing a guidewire, may be used where no large obstruction is indicated. In this method, the guidewire is inserted through the stent only until it is flush with or within the tip of the stent. A pusher is again inserted behind the stent on the guidewire and is locked to the guidewire with a locking hub (e.g. Speed-Lok.RTM. product available from Boston Scientific Corporation, Watertown, Mass.). The assembly is then pushed by the pusher catheter acting on the trailing end to enter the cystoscope and then the ureter.
The choice of technique is based on physician preference and evaluation of the patient. For instance, if the obstruction is small, the physician may first try to use the retrograde technique in which the wire does not extend beyond the entry end of the stent for saving time and cost. But if that technique is unsuccessful, the stent is withdrawn and the guidewire is inserted retrograde. As the wire is much smaller in diameter than the stent it can more easily be negotiated past the obstruction. When the wire is successfully placed, the stent is passed over the wire. The over-the-wire technique is usually more reliable and less traumatic to the patient, and also may lessen the risk of ureteral perforation or puncture.
It is preferable for the hospital to be able to stock one stent unit to be used in both retrograde placement techniques as it involves less inventory cost. Also a dual-use stent allows the physician to have both options when he opens the package. It is therefore highly desirable that a single stent be capable of both types of placement and capable of using a guidewire as large as the common 0.038 inch guidewire.
It is likewise desirable for a stent to carry markings of its identity so that, for instance, a physician, when withdrawing a used stent, can determine e.g. its length, French size and style, to be able to assuredly select a replacement stent of identical character.
Furthermore, it has been found that by using a hydrophilic, dissolving tip on the end of a ureteral stent, significant advantages can be obtained, as are disclosed in U.S. Pat. No. 5,049,138, which is hereby incorporated by reference. In this case two very dissimilar materials are employed with two different desirable attributes. The dissolvable tip is very rigid and hydrophilic (lubricious) which both assist in non-traumatic placement. The body of the catheter to reside in the ureter is very soft and pliable for patient comfort and for avoidance of trauma over the duration of its residence in the ureter. By being dissolvable, the hydrophilic entry tip disappears after it has been useful in the placement of the softer material in the ureter. The dissolution of the tip provides a larger passage for improved drainage.
In respect of long-term patient comfort, peristaltic action of the ureter constantly occurs, in normal function. This produces forces and sensations associated with attempted expulsion of the stent. To diminish these tendencies and improve patient tolerance, it is highly desireable that the stent be as small in diameter as will perform the drainage task. Also, the smaller the stent, the easier it is to pass through the endoscope and pass the ureteral obstruction.
Heretofore, however, it has not been possible to put a dissolving or hydrophilic tip on a stent of the desired small 6 French size, while having the capability to place the stent over the widely preferred wire size of 0.038 inch diameter. Such combination has appeared unachievable because of the dimensional characteristics and requirements of the components.